Most automotive vehicles are propelled by internal combustion-engines consuming hydrocarbon fuels. Burning these fuels produces exhaust gas containing harmful air pollutants, such as carbon monoxide, nitrogen oxides, and unburned hydrocarbons. It also contains substantial amount of carbon dioxide which, if produced in large quantities worldwide over long period of time, can contribute to an undesirable increase in average global temperature. Concern for clean air and a desire to prevent adverse consequences of man-made global warming dictate a need to substantially improve fuel efficiency of automotive vehicles.
By itself, the internal-combustion engine is a reasonably efficient machine. Unfortunately, the driving pattern of most automotive vehicles is such that a substantial fraction of energy produced by their engines is wasted. Typically, the driving pattern involves frequent accelerations, each followed by a deceleration. Each acceleration involves a significant increase in fuel consumption needed to produce the additional energy necessary to increase the vehicle speed. Then, during a subsequent deceleration, this added energy is absorbed by vehicle brakes and dissipated as heat.
Attempts to overcome such waste of energy led to development of systems, in which the energy of vehicle motion is not dissipated during braking but converted into a form in which it can be temporarily stored and, then, used again to accelerate the vehicle at a later time. Typically, such system includes an internal-combustion engine, an energy storage, and a second machine absorbing the energy of vehicle motion and placing it into the storage during braking. During subsequent acceleration, the second machine receives energy, from the energy storage, and uses it to supplement the work of the internal-combustion engine. Such systems are known as hybrid vehicle systems. An electric hybrid includes an electric generator/motor as a second machine and an electric battery for energy storage. A fluid-power hybrid includes a pump/motor and a pressurized-fluid accumulator. A flywheel hybrid includes a variable-ratio transmission and a flywheel. Another type of a hybrid, an air hybrid, does not require a second machine to absorb the energy of vehicle motion. In the air hybrid, the vehicle engine absorbs the braking energy of the vehicle and puts it in storage in the form of compressed air. Later, the compressed air is used to assist the engine in propelling the vehicle.
A disadvantage, common to all of the above mentioned hybrids, is the fact that the improvement in fuel consumption, they bring about, takes place only during the stop-and-go driving conditions, typical for in-city driving. During highway driving, when the vehicle is cruising on the road with approximately constant speed, a hybrid system offers practically no fuel economy advantage. It is clear that it is highly desirable to have a vehicle system that offers a substantial improvement in fuel consumption during cruising with approximately constant speed on a highway, while retaining all the fuel economy advantages of a hybrid system during stop-and-go driving in a city. Such a system is the subject of the present invention.